The impact of MEMS (Microelectromechanical Systems) devices on the microelectronics industry has been extremely significant in recent years. Indeed, MEMS is one of the fastest growing areas of microelectronics. The growth of MEMS has been enabled, to a large extent, by the extension of silicon-based photolithography to the manufacture of micro-scale mechanical devices and structures. Photolithographic techniques, of course, rely on reliable etching techniques, which allow accurate etching of a silicon substrate revealed beneath a mask.
MEMS devices have found applications in a wide variety of fields, such as in physical, chemical and biological sensing devices. One important application of MEMS devices is in inkjet printheads, where micro-scale actuators for inkjet nozzles may be manufactured using MEMS techniques. The present Applicant has developed printheads incorporating MEMS ink ejection devices and these are described in the following patents and patent applications, all of which are incorporated herein by reference.
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Typically a MEMS inkjet printhead (“MEMJET printhead”) is comprised of a plurality of printhead integrated circuits, with each integrated circuit having several thousand nozzles. Each nozzle comprises an actuator for ejecting ink, which may be, for example, a thermal bend actuator (e.g. U.S. Pat. No. 6,322,195) or a bubble-forming heater element actuator (e.g. U.S. Pat. No. 6,672,709). The integrated circuits are manufactured using MEMS techniques, meaning that a high nozzle density and, hence, high resolution printheads can be mass-produced at relatively low cost.
In the manufacture of MEMS printhead integrated circuits, it is often required to perform deep or ultradeep etches to depths of over 10 micron. A problem with deep etches, especially ultradeep etches, is maintaining anisotropy during the etch—that is, ensuring the trench is etched in a vertical direction, but not in a horizontal direction. Ideally, the sidewalls of the trench should be substantially perpendicular with respect to the surface of the substrate.
It is particularly important to have perpendicular sidewalls in ultradeep trenches when etching ink supply channels through silicon wafers. MEMS printhead integrated circuits require delivery of ink to each nozzle through either an individual or a common ink supply channel. These ink channels are typically etched through wafers having a thickness of about 200 micron, and therefore place considerable demands on the ultradeep etching method employed. It is especially important that each ink channel is substantially perpendicular to the wafer surface and does not contain kinks or sidewall projections (e.g. grassing), which can interfere with the flow of ink.
In the Applicant's U.S. patent application Ser. No. 10/728,784 and Ser. No. 10/728,970 , both of which are incorporated herein by reference, there is described a method of fabricating inkjet printheads from a wafer having a drop ejection side and an ink supply side. Referring to FIG. 1, there is shown a typical MEMS nozzle arrangement 1 comprising a bubble-forming heater element actuator assembly 2. The actuator assembly 2 is formed in a nozzle chamber 3 on the passivation layer 4 of a silicon wafer 5. The wafer typically has a thickness “B” of about 200 micron, whilst the nozzle chamber typically occupies a thickness “A” of about 20 micron.
Referring to FIG. 2, an ink supply channel 6 is etched through the wafer 5 to the CMOS metallization layers of an interconnect 7. An inlet 8 provides fluid connection between the ink supply channel 6 and the nozzle chamber (removed for clarity in FIG. 2). CMOS drive circuitry 9 is provided between the wafer 5 and the interconnect 7. The actuator assembly 2, associated drive circuitry 9 and ink supply channel 6 may be formed on and through a wafer 3 by lithographically masked etching techniques, as described in U.S. application Ser. No. 10/302,274, which is incorporated herein by reference.
Referring to FIG. 3, the ink supply channel 6 is formed in the wafer 5 by first etching a trench partially through the wafer 5 from the drop ejection side (i.e. nozzle side) of the wafer. (This trench will become the inlet 8, shown in FIG. 2). Once formed, the trench is plugged with photoresist 10, as shown in FIG. 3, and the ink supply channel 6, is formed by ultradeep etching from the ink supply side of the wafer 5 to the photoresist plug 10. Finally, the photoresist 10 is stripped from the trench to form the inlet 8, which provides fluid connection between the ink supply channel 6 and the nozzle chamber 3.
Alternatively, each ink supply channel may be configured to supply ink to a plurality of nozzles which all eject the same colored ink. This arrangement is illustrated in FIG. 4 and is described in detail in the Applicant's copending application Ser. No. 10/760,254 (Applicant Ref: RRC022), the contents of which is incorporated herein by reference.
In either of these ink supply channel configurations, the “back-etching” technique avoids filling and removing an entire ink supply channel with resist whilst nozzle structures in the wafer are being lithographically formed. Notwithstanding the problems of etching anisotropically to a depth of up to 200 micron, it is also desirable when etching ink supply channels to provide hydrophilic channel sidewalls. Optimum printing conditions in an inkjet printhead are generally achieved by having a hydrophobic nozzle face and hydrophilic ink supply channels. Hydrophilic ink supply channels ensure that the aqueous-based inkjet ink is drawn into the ink supply channels from a bulk ink reservoir. A hydrophobic nozzle face ensures the formation of discrete ink droplets when the ink is ejected from each nozzle and also minimizes surface flooding during printing.
Several methods for etching ultradeep trenches into silicon are known in the art. All these methods involve deep reactive ion etching (DRIE) using a gas plasma. The semiconductor substrate, with a suitable mask disposed thereon, is placed on a lower electrode in a plasma reactor, and exposed to an ionized gas plasma formed from a mixture of gases. The ionized plasma gases (usually positively charged) are accelerated towards the substrate by a biasing voltage applied to the electrode. The plasma gases etch the substrate either by physical bombardment, chemical reaction or a combination of both. Etching of silicon is usually ultimately achieved by formation of volatile silicon halides, such as SiF4, which are carried away from the etch front by a light inert carrier gas, such as helium.
Anisotropic etching is generally achieved by depositing a passivation layer onto the base and sidewalls of the trench as it is being formed, and selectively etching the base of the trench using the gas plasma.
One method for achieving ultradeep anisotropic etching is the “Bosch process” , described in U.S. Pat. Nos. 5,501,893 and 6,284,148. This is the current method of choice in commercial MEMS foundries and involves alternating polymer deposition and etching steps. After formation of a shallow trench, a first polymer deposition step deposits a polymer onto the base and sidewalls of the trench. The polymer is deposited by a gas plasma formed from a fluorinated gas (e.g. CHF3, C4F8 or C2F4) in the presence or in the absence of an inert gas. In the subsequent etching step, the plasma gas mix is changed to SF6/Ar. The polymer deposited on the base of the trench is quickly broken up by ion assistance in the etching step, while the sidewalls remain protected. Hence, anisotropic etching may be achieved. However, a major disadvantage of the Bosch process is that polymer deposition and etching steps need to be alternated, which means continuously alternating the gas composition of the plasma. This alternation, in turn, leads to slow etch rates and uneven trench sidewalls, characterized by scalloped surface formations. Plasma instability as the gas chemistry is switched also tends to exacerbate the formation of uneven sidewalls.
Moreover, the Bosch etch leaves a hydrophobic polymer coating on trench sidewalls. As discussed above, hydrophobic sidewalls are undesirable in fluidics applications, such as ink supply channels for inkjet printheads. Accordingly, in inkjet printhead applications, the Bosch etch is usually followed by a post-etch cleaning process, such as EKC wet cleaning, dry O2 plasma ashing or combinations thereof The post-etch cleaning process is intended to remove the hydrophobic polymer and leave channel sidewalls coated with SiO2. However, post-etch cleaning processes undesirably add to the number of fabrication steps and can also lead to their own inherent problems, such as wafer-cracking during EKC cleaning.
A modification of the cyclical Bosch process is described in U.S. Pat. No. 6,127,278, assigned to Applied Materials, Inc. In the Applied Materials process, a first passivation etch is performed using a HBr/O2 plasma, followed by a main etch using a SF6/HBr/O2 in alternating succession. The HBr enhances passivation, probably by formation of relatively nonvolatile silicon bromides in the passivation layer. However, the problem of hydrophobically coated sidewalls still remains with the Applied Materials process.
In order to avoid the cumbersome Bosch process, in which plasma gases need to be continuously alternated, anisotropic etching techniques were developed, which use simultaneous sidewall passivation. In such etching methods, a plasma mix is formed from a passivating component and an etching component. A typical plasma mix is formed from O2/SF6 with the addition of He as a carrier gas being highly recommended to enhance ion dispersement. The plasma mix simultaneously passivates and etches, which avoids the disadvantages of-the Bosch process. Nevertheless, it is the general view that mixing the gases gives less effective anisotropic etching, because the two processes tend to be self-cancelling. Accordingly, simultaneous sidewall passivation etching has been mostly confined to etching relatively shallow trenches. For ultradeep anisotropic etching, alternating passivation/etching is by far the most preferred technique.
One successful process for etching ultradeep trenches, which does not require alternating plasma gas mixtures, is the “Lam process” described in U.S. Pat. No. 6,191,043. In the Lam process, a passivating/etching plasma is formed from a mixture of O2, SF6, He and Ar—the O2 is a passivating gas; the SF6 is an etching gas; the He is a carrier gas; and the Ar is a bombardment-enhancing gas. Trench depths of up to 60 micron have been reported using the Lam process with acceptable etch rates. However, the process has not been used widely and etch depths of greater than 60 micron have not been reported.
None of the above-described etch processes can be used to etch trenches through a typical wafer to a depth of over 100 micron, whilst leaving hydrophilic sidewalls. Even when the etch process (or post-etch treatment) leaves SiO2-coated sidewalls, these SiO2-coated sidewalls are not particularly hydrophilic, having a contact angle of about 60°. Truly hydrophilic surfaces have contact angles of less than 50°, preferably less than 40° or preferably less than 30°.
It would be desirable to provide a new reactive ion etching process, which is capable of anisotropically etching ultradeep trenches of over 100 micron. It would be particularly desirable for the process to leave hydrophilic sidewalls after the etch, without the need for any post-etch hydrophilization treatments.